Heat transfer system and method

ABSTRACT

A heat transfer system including a first heat exchange assembly, a second heat exchange assembly, and a heat transfer circuit interconnecting the first and second heat exchange assemblies, the circuit incorporating a working fluid to transfer heat between the first and second heat exchange assemblies. The heat transfer circuit preferably takes the form of a closed loop heat-pipe system. The heat transfer system is operative in a first mode where the working fluid is at or below a threshold pressure and in a second mode where the working fluid is at a higher pressure than the threshold pressure. A pressure regulating device is also provided to increase the pressure of the working fluid above the threshold pressure to effect change of operation of the heat transfer system from the first mode to the second mode. Optionally the pressure regulating device is a pressure vessel having at least one heat transfer surface. An air-conditioning system for conditioning air in a region, including the heat transfer system wherein the first heat exchange assembly is operative to moderate air temperature in the region and wherein the second heat exchange assembly is disposed adjacent a source/sink of substantially constant temperature.

TECHNICAL FIELD

This disclosure relates generally to heat transfer systems and methodsof operating heat transfer systems, especially passive heat transfersystems employing loop heat-pipes. Particularly, although notexclusively, the heat transfer system and method are suited toconditioning air in a region, such as a habitable internal space.

BACKGROUND

Approximately forty percent of world energy usage is associated withdomestic and commercial buildings and in developed countries asignificant portion of this energy usage is involved in the heating andcooling systems including the energy required by air-conditioningsystems. Savings in energy in all areas, including these, is rapidlybecoming an important element in the adoption of renewable energysources to address climate change.

SUMMARY

In one embodiment, there is provided a heat transfer system including afirst heat exchange assembly, a second heat exchange assembly, and aheat transfer circuit interconnecting the first and second heat exchangeassemblies, the circuit incorporating a working fluid to transfer heatbetween the first and second heat exchange assemblies. The heat transfercircuit may take the form of a closed loop heat-pipe system but is notlimited to such an arrangement. The heat transfer system is operative ina first mode where the working fluid is at or below a threshold pressureand in a second mode where the working fluid is at a higher pressurethan the threshold pressure. A pressure regulating device is alsoprovided to increase the pressure of the working fluid above thethreshold pressure to effect change of operation of the heat transfersystem from the first mode to the second mode. Optionally the pressureregulating device is a pressure vessel having at least one heat transfersurface.

In another embodiment, there is provided an air-conditioning system forconditioning air in a region, including a heat transfer system having afirst heat exchange assembly operative to moderate air temperature inthe region and a second heat exchange assembly disposed adjacent asource/sink of substantially constant temperature. A fluid circuitincluding a working fluid transfers heat between the first and secondheat assemblies and a pressure regulating device is also provided toincrease the pressure of the working fluid. In another embodiment, apressure regulating device for a heat transfer system includes apressure vessel with at least one heat transfer surface. Optionally thepressure vessel is coupled by a valve to a working fluid circuit of aheat transfer system.

In another embodiment, a method of controlling a heat transfer system isprovided. The system is operative in a first mode where the workingfluid is at or below a threshold pressure and in a second mode where theworking fluid is at a higher pressure than the threshold pressure. Themethod includes regulating working fluid pressure in a heat transfercircuit containing the working fluid by increasing the pressure of theworking fluid above the threshold pressure to effect change of the heattransfer system from the first mode to the second mode.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the accompanying drawings and thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an illustrative embodiment of aheat transfer system.

FIG. 2A is a schematic representation showing the heat transfer systemof

FIG. 1 in a heating mode.

FIG. 2B is a schematic representation showing the heat transfer systemof FIG. 1 in a cooling mode.

FIG. 3 is a detailed schematic representation of a heat transfer systemof a further embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented herein. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe drawing figures, can be arranged, substituted, combined, separated,and designed in a wide variety of different configurations, all of whichare explicitly contemplated herein.

This disclosure is directed generally to heat transfer systems andmethods of operating heat transfer systems, especially passive heattransfer systems employing loop heat-pipes. While the disclosure isdescribed generally in the context of systems to condition air in aregion, such as a habitable internal space, it is not limited to suchinstallations, and may be used in other applications.

Heat transfer systems for application to habitable internal spaces, suchas domestic dwellings and commercial buildings, have employed heat pumpsystems which consume energy or solar passive arrangements. One priorart example involves controlling the insolation and radiation of passivesolar thermal storage columns for heating and cooling of homes and otherstructures. The example includes rotatable insulating panels whichcontrol the exposure of thermal storage columns to daytime sunlight andthe nighttime sky.

Whilst there are examples of heat-pipe based heat transfer systems, thetypical focus is on cooling applications for electrical equipment andfor satellite, space vehicle or similar micro-gravity applications.These heat transfer solutions are not appropriate for application tohabitable internal spaces, such as required for domestic dwellings orcommercial buildings.

Disclosed in some embodiments is a heat transfer system including afirst heat exchange assembly; a second heat exchange assembly; a heattransfer circuit interconnecting the first and second heat exchangeassemblies. The circuit includes a working fluid to transfer heatbetween the first and second heat exchange assemblies. The heat transfersystem has a first mode where the working fluid is at or below athreshold pressure and a second mode where the working fluid is at ahigher pressure than the threshold pressure. The heat transfer systemfurther includes a pressure regulating device to increase the pressureof the working fluid above the threshold pressure to effect change ofthe heat transfer system from the first mode to the second mode.

Also disclosed in some embodiments is the heat transfer circuit in theform of a closed loop heat-pipe system. In some forms, the first andsecond heat transfer assemblies may each include a wicking material. Insome embodiments, the first and second heat transfer assemblies may beoperative mutually alternatively as evaporators and condensers.

Disclosed in some embodiments, the working fluid is arranged to transferheat to the first heat exchange assembly from the second heat exchangeassembly in the first mode. In some embodiments, the working fluid isarranged to transfer heat to the second heat exchange assembly from thefirst heat exchange assembly in the second mode.

Disclosed in some embodiments, the pressure regulating device comprisesa pressure vessel having at least one heat transfer surface. In someforms, the pressure vessel may be coupled to the heat transfer circuitvia a fluid flow control valve. In some embodiments, the working fluidmay be selected from the group including demineralised water, ammonia,acetaldehyde, ether, pentane, ethyl chloride, and refrigerants R-245fa(1,1,1,3,3 Pentafluoropropane) and its substitutes.

Disclosed in some embodiments is an air-conditioning system forconditioning air in a region including a heat transfer system having afirst heat exchange assembly operative to moderate air temperature inthe region and a second heat exchange assembly is disposed adjacent asource/sink of substantially constant temperature. The heat transfersystem may be in any form as described above. Disclosed in someembodiments, the pressure regulating device may use a pressure sourcebased on temperature of a separate region.

Disclosed in some embodiments is an air-conditioning system forconditioning air in a habitable internal space including a first heatexchange assembly to moderate the temperature of air in the habitableinternal space, a second heat exchange assembly adjacent a source ofsubstantially constant temperature; and a pressure vessel having atleast one heat transfer surface exposed to ambient temperatureconditions.

Disclosed in some embodiments, the source/sink of substantially constanttemperature is a subterranean location below the habitable internalspace. The subterranean location may be in the range of 1 to 5 metresbelow ground surface, preferably 2 to 3 metres below the surface. Insome embodiments, the second heat exchange assembly may beground-coupled at the subterranean location. Disclosed in some forms,the first threshold pressure is established so that in the first modethe second heat exchange assembly acts as an evaporator at thesubstantially constant temperature to allow heat transfer from thesecond heat exchange assembly to the first heat exchange assembly. Insome forms in the second mode, the pressure of the working fluid is at alevel above the threshold pressure such that the second heat exchangeracts as a condenser at the substantially constant temperature to allowheat transfer from the first heat exchange assembly to the second heatexchange assembly.

Disclosed in some forms is a pressure regulating device having apressure vessel with at least one heat transfer surface. Optionally avalve may be included for coupling the pressure vessel to a workingfluid circuit of the heat transfer system. Disclosed in some forms is amethod of controlling a heat transfer system, the system being operativein a first mode where the working fluid is at or below a thresholdpressure and a second mode where the working fluid is at a higherpressure than the threshold pressure, the method including regulatingworking fluid pressure in a heat transfer circuit containing the workingfluid by increasing the pressure of the working fluid above thethreshold pressure to effect change of the heat transfer system from thefirst mode to the second mode.

In some form the method further includes selectively reducing thepressure of the working fluid to at or below the threshold pressure torevert the operation of the heat transfer system from the second mode tothe first mode. A pressure regulating device may be provided to regulateworking fluid pressure. In some forms, the pressure regulating devicemay be coupled to a working fluid circuit via a valve to enable theselective pressure regulation of the working fluid.

As illustrated in the Figures, some illustrative embodiments of a heattransfer system is suitable for use in conditioning air in a habitablespace using the ground as a heat source or sink and ambient temperatureto regulate pressure of a working fluid in system. FIG. 1 is a schematicrepresentation of an illustrative embodiment of a heat transfer system100. The design of the heat transfer system 100 as illustrated is basedon a reversible looped heat pipe that connects a heat sink or sourceservicing a region, such as an internal space in a building, in acircuit to a sink or source of constant, near or substantially constanttemperature. One such source is a subterranean location some 1 to 5metres deep, typically located about 2 or 3 metres below ground surface.The system may include a heat transfer circuit suitably in the form ofclosed loop heat-pipe system where a working fluid, such asdemineralised water, evaporates at a hotter end and condenses at acooler end thus transferring heat from the hot region to the coolerregion.

In a typical temperate climatic zone the temperature at a location 2 or3 metres underground is a near constant 15° C. When the temperature inthe building is above this value the heat-pipe system can be configuredand operated to cool the building by extracting heat and dissipating itunderground. Similarly, during winter when the building internaltemperature goes below 15° C. the heat-pipe system can be reversed toact to heat the building internal space.

The heat transfer system 100 includes a first heat exchange assembly 101and a second heat exchange assembly 102, and a heat transfer circuit 103which interconnects the two heat exchangers. The heat transfer circuit103 contains a working fluid (not shown), which exists in a vapour phaseand/or a in liquid phase, in order to transfer heat between the firstheat exchange assembly 101 and second 102 heat exchange assembly. Theheat transfer circuit includes two pipeline sections 104, 105 whichcarry the working fluid and couple respective ends of the first andsecond heat exchangers. The heat transfer system 100 also includes apressure regulating device, here in the form of a pressure chamber orvessel 106, which is coupled to the heat transfer circuit 103 by a fluidflow control valve 107.

The embodiment employs a reversible heat-pipe system which depends onlyon the atmospheric temperature for setting the direction of flow ofworking fluid in the heat transfer circuit 103. The heat transfer systemof the embodiment is applied in an air-conditioning system arranged tocondition air in a region, here the air in a habitable internal space111 enclosed by a building 110. The first heat exchanger 101 isoperative to moderate the temperature of the air in the habitableinternal space 111, for example to a typical desired temperature ofapproximately 22° C., within the building 110. The typical temperaturerange of atmospheric air of the building's immediate externalenvironment, here located in a temperate climatic zone, is from 40° C.to 5° C. The second heat exchanger 102 is disposed adjacent a source 112of substantially constant temperature, here being about 2 to 3 metresbelow ground level 113. In a typical temperate climatic zone, thetemperature 2 or 3 metres underground is a near constant approximately15° C.

When the internal air temperature within the building is above theapproximate 15° C. value, typical of summer, the heat-pipe system 100 ofthe embodiment can be operated to cool internal air in the building 110by extracting heat from the internal habitable space 111 and dissipatingit underground. Similarly, during winter when the building internalambient temperature goes below 15° C. the heat-pipe system can bereversed to act to heat air in the habitable internal space 111 of thebuilding 110. The first and second heat exchangers in this embodimentcan operate as either condensers or evaporators, as required, preferablyin a substantially mutually alternative arrangement.

The reversing capability of the heat transfer system 100 of theembodiment is achieved by regulating the pressure of the working fluidwithin the heat transfer circuit 103, including the piping sections 104,105. For example, when water is the working fluid at a pressure of about3.5 kPa water boils at 25° C. and when the pressure is reduced to about2.0 kPa water boils at 15° C. A passive pressurising/de-pressurisingdevice is included in the loop heat-pipe arrangement, namely thepressure vessel 106. The vessel 106 is a sealed chamber where oneheat-absorbing surface 109 is exposed to the atmosphere. The vessel'schamber is connected to the heat-pipe by the control valve 107. Bycontrolling the working fluid pressure in the heat transfer circuit 103the heating-cooling operating cycle or mode can be reversed. When theatmospheric temperature rises to a threshold value, the control valve107 opens on the pressure vessel 106 connected to the heat pipe portion104. This equalises pressures in each of the pressure vessel 106 andheat transfer circuit 103, thus increasing the heat pipe 104 pressure.This pressure increase acts to reverse the direction of flow of theworking fluid and thus the heat transfer direction in the heat pipe fromcooling to heating, as will be explained further in relation to FIGS. 2Aand 2B. The exposed surface 109 of the pressure vessel 106 is arrangedto absorb and discharge heat based on the external ambient temperature.The vessel 106 is placed outside the building 110, but is desirablyprotected from direct sunlight and shielded as much as is practical fromwind and other environmental influences so that only the ambient airtemperature affects the heat transfer in either direction across surface109.

FIG. 2A is a schematic representation showing the heat transfer system100 in a heating mode. With an external ambient temperature lower thanthe below ground temperature, e.g. in the range of 5° C. to 10° C. andthe control valve 107 closed, the initial working fluid pressure is setto a level where the heat transfer system 100 is in a heating mode, asdepicted in FIG. 2A. In the heating mode, the first heat exchangeassembly is operating as a condenser 101.1 for heating air in theinternal space 111 and the below-ground heat exchanger is operating aevaporator 102.1, at 15° C. In the heating mode the heat transfer fluidin pipe section 104 is substantially in the vapour phase and flows fromthe second heat exchange assembly/evaporator 102.1 to the first heatexchange assembly/condenser 101.1. The heat transfer fluid in pipingsection 105 is substantially in the liquid phase and flows from thefirst heat exchange assembly/condenser 101.1 to the second heat exchangeassembly/evaporator 102.1, as depicted by the arrow in FIG. 2A.

When the external ambient temperature rises above the constant belowground 112 temperature, e.g. in the range of 25° C. to 40° C., thisincreases the pressure within the pressure vessel 106, including by heattransfer across the surface 109. When the control valve 107 is opened,pressures equalise across the pressure vessel 106 and the heat transfercircuit 103, including in the heat-pipe section 104. When the workingfluid pressure in the heat transfer circuit 103 increases above apre-determined threshold, the heat transfer system will change operatingmodes and the flow of working fluid and thus the heating/cooling cyclewill be reversed. This changed operating mode resulting from opening ofthe control valve 107 is depicted in FIG. 2B.

FIG. 2B is a schematic representation showing the heat transfer system100 in a cooling mode. In the cooling mode, the first heat transferassembly operates as an evaporator 101.2 to absorb heat and thusmoderate the temperature in the internal space 111 of building 110towards the desired 22° C. Heat exchange fluid in the vapour phase flowsin pipe section 104 from the first heat exchanger to the second heatexchange assembly which now operates as a condenser 102.2 at nearconstant 15° C., as depicted by the arrow in FIG. 2B. The pipe section105 now carries condensed heat exchange fluid in liquid phase from thesecond heat exchanger back to the first in pipe section 105, to completethe heat transfer circuit 103.2.

One skilled in the art will appreciate that, for this and otherprocesses and methods disclosed herein, the functions performed in theprocesses and methods may be implemented in differing order. Forexample, it will be appreciated that by selectively reducing thepressure of the working fluid in the heat transfer circuit 103 to at orbelow the threshold pressure allows the heat transfer system 100 torevert from the second mode to the first operating mode. Furthermore,the outlined steps and operations are only provided as examples, andsome of the steps and operations may be optional, combined into fewersteps and operations, or expanded into additional steps and operationswithout detracting from the essence of the disclosed embodiments.

FIG. 3 is a detailed schematic representation of a heat transfer systemof a further embodiment. The heat transfer system 200 of the furtherembodiment includes a reversible heat-pipe arrangement with first andsecond heat exchange assemblies 201, 202 that can act optimally as bothevaporators and as condensers. Also, for reversibility, where each heatexchanger needs to act as an evaporator as well as a condenser in eachof the operating modes, the structure for both heat exchangers ispreferably similar or even the same.

The heat exchange assemblies 201, 202 are both of heat-pipe design andinclude a wick 208 arranged adjacent at least a portion of the innersurface of the heat-pipe wall. For a reversible heat-pipe, the wickmaterial should have small enough pore size in order to generatesufficient capillary pressure to allow the liquid-vapour flow of workingfluid within the heat-pipe to be maintained in a 2 to 3 metre gravityfield. This gravity field results from the vertical separation due toground-coupling of the second heat exchange assembly 202 at asubterranean location 212 of substantially constant temperature. Wickconstruction materials including nickel, copper and titanium are likelyto satisfy these requirements.

A heat transfer circuit 203 is formed by the first and second heat-pipeexchangers that are coupled together by pipe sections 204, 205 as shownin FIG. 3. The pipe section 204 is further coupled to a pressure vessel206 by a fluid valve 207, which pressure vessel incorporates at leastone heat transfer surface 209. The heat transfer surface 209 is exposedto external ambient temperature conditions in order to effect reversalof operating mode, as described in relation to the embodiment of FIG. 1.In FIG. 3, a first operating mode wherein heat is absorbed by the firstheat exchanger 201 is represented by the arrows in solid outline; whilsta second operating mode wherein heat is radiated by the first heatexchanger 201 is represented by the arrows in dashed outline, includingfor the heat fluid phases in pipe sections 204, 205 linking to thesecond heat exchanger 202.

The material of the wick 208, its effective pore radius, thepermeability and thermal conductivity determine the level of thecapillary pressure achieved in the heat-pipe system. The smaller thepore size the greater the capillary pressure. The smaller this pore sizeis, however, the larger the flow resistance which slows the rate offluid/vapour flow through the system. The capillary pressure isdeveloped when the evaporator section, by absorbing heat, allows thecold liquid to transfer through the wick to be converted to vapour whichflows to the condenser while effectively acting as a “thermal andhydraulic lock” to prevent the heated vapour from mixing with theliquid.

For efficient operation of the heat transfer system, the capillarypressure developed in the wick Δp has to be greater than the sum of thepressure losses due to gravity ΔPg, and due to pressure losses arisingin the liquid/vapour lines as a result of flow resistance.

Another factor in the efficient operation of the heat transfer system isthe characteristics of the working fluid both in the liquid and vapourphases. For example, the heat flow rate through the system depends ofsuch properties of the working fluid as latent heat of evaporation, aswell as on the inner and outer radii of the wick and its thermalconductivity.

The pressurising/depressurising chamber or vessel 206 is linked to theheat transfer circuit 203 and operates according to the external ambienttemperature T where the pressure P in the chamber can be modelledaccording the standard gas law:

P=RT/V

-   -   where the volume V is fixed and the universal gas constant R is        also fixed for a given gas/vapour. Thus, as the ambient        temperature T rises the pressure in the chamber also rises and        hence, when valve 207 is opened, the pressure in the heat-pipe        also rises. The parameters of the chamber, the working fluid and        the dimensional and thermodynamic characteristics of the        heat-pipe can be designed to allow the system to operate        efficiently using the near-constant temperatures existing below        ground.

It is believed that heat transport rates of 70 W/° C. to 290 W/° C. canbe achieved depending on some of the above described parameters. Withammonia as the working fluid, maximum transport capability of about 800W may achieved. The evaporation temperature as well as the temperaturedifferences between evaporation and condensation can also affect thesevalues and generally the larger the temperatures differences the moreefficient is the heat transfer.

Since the direction of heat flow is primarily dependent on the pressureof working fluid in the heat-pipe and the pressure is dependent on theexternal ambient temperature, the system is passive since it does notrequire energy input to control the process. Furthermore, theliquid-vapour flow of working fluid is controlled by capillary pressure,which is a function of the physical design of the heat-pipe includingthe wick material, the vapour and liquid channels and again thispressure rises in a passive manner without using external energy.

A desirable property of any working fluid to achieve reversibility ofthe system is the capability of its boiling point to be changed bychanging the pressure. This property allows the operating mode of thesystem to be reversed by changing the pressure in the system and therebychanging the boiling point from being at or below the ground temperature(heating cycle) to being at or below the set room temperature (coolingcycle). Suitable working fluids, such as water, ammonia, acetaldehyde,ether, pentane, ethyl chloride, and refrigerants R-245fa (1,1,1,3,3Pentafluoropropane) propylene, nitrogen, freon and its substitutes, mayhave boiling points close to the estimated ground temperature at normalatmospheric pressure. This allows relatively small changes in pressureto move the boiling point above or below the ground temperature asrequired for reversibility.

In embodiments suited to domestic housing applications, the proposedheat-pipe system can be implemented by incorporating the heat exchangerinto the floor or walls of the internal space of the house. Theunderground or ground-coupled heat exchanger may be of a similar designwhere it is implemented to maximise the efficiency of the dissipation orabsorption of heat from the ground. The piping connecting the abovesurface and below-surface heat exchangers can also take a number ofdifferent forms depending on the local situation as well as on designdetails to achieve the highest possible capillary pressure.

Certain disclosed embodiments take advantage of constant groundtemperatures as a source of renewable energy to drive a reversible heatpipe for assisting both heating in cold ambient temperatures and coolingin hot ambient conditions, passive control of the reversing processwhich involves consumption of very little or no electrical energy. Someembodiments show potential for implementation in temperate as well assub-tropical environments where ground temperatures remain constant atrelatively low depths, together with the ability to use low-cost workingfluids such as water in some applications.

The present disclosure is not to be limited in terms of the particularembodiments described in this application, which are intended asillustrations of various aspects. Many modifications and variations canbe made without departing from its spirit and scope, as will be apparentto those skilled in the art. Functionally equivalent methods andapparatuses within the scope of the disclosure, in addition to thoseenumerated herein, will be apparent to those skilled in the art from theforegoing descriptions. Such modifications and variations are intendedto fall within the scope of the appended claims. The present disclosureis to be limited only by the terms of the appended claims, along withthe full scope of equivalents to which such claims are entitled. It isto be understood that this disclosure is not limited to particularmethods, reagents, compounds, compositions or biological systems, whichcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular embodimentsonly, and is not intended to be limiting.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted systems are merelyexamples, and that in fact many other systems can be implemented whichachieve the same functionality. In a conceptual sense, any arrangementof components to achieve the same functionality is effectively“associated” such that the desired functionality is achieved. Hence, anytwo components herein combined to achieve a particular functionality canbe seen as “associated with” each other such that the desiredfunctionality is achieved, irrespective of architectures or intermedialcomponents. Likewise, any two components so associated can also beviewed as being “operably connected”, or “operably coupled”, to eachother to achieve the desired functionality, and any two componentscapable of being so associated can also be viewed as being “operablycouplable”, to each other to achieve the desired functionality. Specificexamples of operably couplable include but are not limited to physicallymateable and/or physically interacting components.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should be interpreted to mean “at least one”or “one or more”); the same holds true for the use of definite articlesused to introduce claim recitations. In addition, even if a specificnumber of an introduced claim recitation is explicitly recited, thoseskilled in the art will recognize that such recitation should beinterpreted to mean at least the recited number (e.g., the barerecitation of “two recitations,” without other modifiers, means at leasttwo recitations, or two or more recitations). Furthermore, in thoseinstances where a convention analogous to “at least one of A, B, and C,etc.” is used, in general such a construction is intended in the senseone having skill in the art would understand the convention (e.g., “asystem having at least one of A, B, and C” would include but not belimited to systems that have A alone, B alone, C alone, A and Btogether, A and C together, B and C together, and/or A, B, and Ctogether, etc.). In those instances where a convention analogous to “atleast one of A, B, or C, etc.” is used, in general such a constructionis intended in the sense one having skill in the art would understandthe convention (e.g., “a system having at least one of A, B, or C” wouldinclude but not be limited to systems that have A alone, B alone, Calone, A and B together, A and C together, B and C together, and/or A,B, and C together, etc.). It will be further understood by those withinthe art that virtually any disjunctive word and/or phrase presenting twoor more alternative terms, whether in the description, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” will be understood to include thepossibilities of “A” or “B” or “A and B.”

As will be understood by one skilled in the art, for any and allpurposes, such as in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” and the like include the number recited andrefer to ranges which can be subsequently broken down into subranges asdiscussed above. Finally, as will be understood by one skilled in theart, a range includes each individual member. Thus, for example, a grouphaving 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, agroup having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells,and so forth.

From the foregoing, it will be appreciated that various embodiments ofthe present disclosure have been described herein for purposes ofillustration, and that various modifications may be made withoutdeparting from the scope and spirit of the present disclosure.Accordingly, the various embodiments disclosed herein are not intendedto be limiting, with the true scope and spirit being indicated by thefollowing claims.

1. A heat transfer system comprising: a first heat exchange assembly; asecond heat exchange assembly; a heat transfer circuit interconnectingthe first and second heat exchange assemblies, the circuit incorporatinga working fluid to transfer heat between the first and second heatexchange assemblies; the heat transfer system being operative in a firstmode where the working fluid is at or below a threshold pressure and asecond mode where the working fluid is at a higher pressure than thethreshold pressure; and a pressure regulating device operative toincrease the pressure of the working fluid above the threshold pressureto effect change of the heat transfer system from the first mode to thesecond mode.
 2. The heat transfer system of claim 1 wherein the heattransfer circuit comprises a closed loop heat-pipe system.
 3. The heattransfer system of any preceding claim wherein the first and second heattransfer assemblies each include a wicking material.
 4. The heattransfer system of any preceding claim wherein the first and second heattransfer assemblies are operative mutually alternatively as evaporatorsand condensers.
 5. The heat transfer system of any preceding claimwherein the working fluid is arranged to transfer heat to the first heatexchange assembly from the second heat exchange assembly in the firstmode.
 6. The heat transfer system of any preceding claims wherein theworking fluid is arranged to transfer heat to the second heat exchangeassembly from the first heat exchange assembly in the second mode. 7.The heat transfer system of any preceding claim wherein the pressureregulating device comprises a pressure vessel having at least one heattransfer surface.
 8. The heat transfer system of claim 7 wherein thepressure vessel is coupled to the heat transfer circuit via a fluid flowcontrol valve.
 9. The heat transfer system of any preceding claimwherein the working fluid is selected from the group includingdemineralised water, ammonia, acetaldehyde, ether, pentane, ethylchloride, and refrigerants R-245fa (1,1,1,3,3 Pentafluoropropane). 10.An air-conditioning system for conditioning air in a region, comprisingthe heat transfer system of any one of claims 1 to 9 wherein the firstheat exchange assembly is operative to moderate air temperature in theregion and wherein the second heat exchange assembly is disposedadjacent a source/sink of substantially constant temperature.
 11. Theair-conditioning system of claim 10 wherein the pressure regulatingdevice uses a pressure source based on temperature of a separate region.12. An air-conditioning system for conditioning air in a habitableinternal space, comprising the heat transfer system of either claim 7 orclaim 8 wherein: the first heat exchange assembly is operative tomoderate the temperature of air in the habitable internal space; thesecond heat exchange assembly is disposed adjacent a source ofsubstantially constant temperature; and the at least one heat transfersurface of the pressure vessel is exposed to ambient temperatureconditions.
 13. The air-conditioning system of either claim 10 or claim11 wherein the source/sink of substantially constant temperature is asubterranean location below the habitable internal space.
 14. Theair-conditioning system of claim 12 wherein the subterranean location isin the range of 1 to 5 metres below ground surface, preferably 2 to 3metres below the surface.
 15. The air-conditioning system of eitherclaim 12 or claim 13 wherein the second heat exchange assembly isground-coupled at the subterranean location.
 16. The air-conditioningsystem of any one of claims 12 to 15, wherein the first thresholdpressure is established so that in the first mode the second heatexchange assembly is capable of acting as an evaporator at thesubstantially constant temperature to allow heat transfer from thesecond heat exchange assembly to the first heat exchange assembly. 17.The air-conditioning system of any one of claims 12 to 15, wherein, whenin the second mode, the pressure of the working fluid is at a levelabove the threshold pressure such that the second heat exchanger iscapable of acting as a condenser at the substantially constanttemperature to allow heat transfer from the first heat exchange assemblyto the second heat exchange assembly.
 18. A pressure regulating devicefor a heat transfer system comprising a pressure vessel with at leastone heat transfer surface.
 19. The pressure regulating device of claim18 further comprising a valve for coupling the pressure vessel to aworking fluid circuit of the heat transfer system.
 20. A method ofcontrolling a heat transfer system, the system being operative in afirst mode where the working fluid is at or below a threshold pressureand a second mode where the working fluid is at a higher pressure thanthe threshold pressure, the method comprising: regulating working fluidpressure in a heat transfer circuit containing the working fluid byincreasing the pressure of the working fluid above the thresholdpressure to effect change of the heat transfer system from the firstmode to the second mode.
 21. The method of claim 20 further comprisingselectively reducing the pressure of the working fluid to at or belowthe threshold pressure to revert the operation of the heat transfersystem from the second mode to the first mode.
 22. The method of eitherclaim 20 or claim 21 wherein a pressure regulating device is provided toregulate working fluid pressure.
 23. The method of claim 22 wherein apressure regulating device is provided to regulate working fluidpressure and is coupled to a working fluid circuit via a valve to enablethe selective pressure regulation of the working fluid.